Peri-Gondwanide Palaeozoic Volcanic Ash Beds

Ninety-five K-bentonite levels have been recorded to date from the Upper Ordovician (Ashgill) to Lower Devonian (Lochkov) sequences of the Carnic Alps. They occur in shallow to deep water fossiliferous marine sediments which suggest a constant movement from a moderately cold climate of approximately 50 southern latitude in the Upper Ordovician to the Devonian reef belt of some 30 south. The reconstructed distribution of the various litho- and biofacies indicates a SW-NE directed polarity from shallow water environments to an open marine and deep sea setting. Samples from the classic sections of (1) Seewarte, (2) Cellon, (3) Oberbuchach, (4) Dr Steinwender Hutte, (5) Nölbling Graben (6) Uggwa, (7) Valbertad and (8) Hoher Trieb which are stratigraphically well-dated by graptolites and conodonts were studied in detail. Immobile trace elements and REE have been used to provide information on the magmatic composition of K- Bentonite parent ashes and tectonic setting of the source volcanoes. The geochemical analysis of the samples taken indicate that the Cellon and Seewarte sections plot within the ocean ridge field, the Oberbuchach, Nölbling Graben, Dr Steinwender Hütte, Uggwa, Valbertad and Hoher Trieb sections mainly plot within the volcanic arc and syn- collisional fields. The samples vary in composition and plot mainly within the andesite, rhyodacite/dacite fields. The abundant presence of these K-bentonite horizons in the Llandovery - Middle Ludlow sequences of the Carnic Alps is similar in the British Isles, Sweden, Canada and N. America where they document widespread volcanism related to the closing of the Iapetus Ocean and northward drifting of microplates derived from the northern margin of Gondwana. As the K-bentonite horizons in the Carnic Alps range from a few millimeters to 2-3 centimeters maximum thickness this may indicate that the volcanic source area must have been quite distant. We thus conclude that the majority of the K-bentonites found in the Carnic Alps during this period were rather derived from neighbouring peri-Gondwanide terranes, such as the Hun Terrane, than from far distant sources at the eastern margin of the closing Iapetus ocean. Silurian K-Bentonites which are Pridoli in age may be comparable with those described from Podolia for which a source area in the Rheic Ocean has been indicated.